Much ado about Pluto.

Three generations of schoolchildren have learned that Pluto is a planet. After considerable uproar over whether this tiny member of the solar system should be called something else, the International Astronomical Union 0AU) has decided to let Pluto retain its noble rank.

Brian G. Marsden says it was never his intent to demote Pluto. But that's not the way many interpreted his proposal last year that this icy orb, discovered in 1930, belongs in a catalog of objects known as the minor planets. The category includes such solar system detritus as asteroids and inactive comets.

Marsden, a planetary scientist at the Smithsonian Astrophysical Observatory in Cambridge, Mass., emphasizes that his proposal would not have stripped Pluto of its planetary status. The classification, he says, simply reflects the fact that Pluto has much in common with denizens of the Kuiper belt, the reservoir of icy objects that lies beyond the orbits of Neptune and Pluto and is thought to be a main source of comets. Although Pluto is 10 times as big as any known member of the Kuiper belt, its composition and orbit are similar to objects in the reservoir.

In contrast, Pluto is an oddball among its eight sister planets. It's the smallest in both size and mass, and has the most elliptical orbit. It moves in a plane tilted markedly away from the other planets' orbits. Moreover, Pluto is the only planet made almost entirely of ice.

In Marsden's plan, Pluto would have become the largest object in the minor-planet catalog, a title held for 198 years by the asteroid Ceres. in recognition of its special status, Pluto would also have been designated as the catalog's 10,000th member.

"It's a bad idea," says astronomer Richard P. Binzel of the Massachusetts Institute of Technology. "Scientifically and semantically, Pluto can't be a major planet and a minor planet at the same time.... How would you like to be known as the 10,000th object? ... You'd have to rewrite history." IAU rejected Marsden's proposal in a statement Feb. 3.

Arguing that Pluto has characteristics of both planets and the Kuiper belt, Binzel initially suggested a counterproposal: Keep Pluto as a planet but make it the charter member of a new catalog composed exclusively of Kuiper belt objects, in recognition of expanded knowledge of the outer solar system, Binzel still favors creating such a catalog. But after the debate over the orb's fate reached a fevered pitch in January, he now concludes that astronomers should "just leave Pluto alone."